It is well known that the relationship between pulse and respiratory rates may serve as an indicator of a subject's psychological or physical health. For example, in the case of illness, the pulse rate/respiratory rate ratio may increase or decrease.
The following is known about the relationship between the respiratory rate and the physiological state. Namely:
(i) There is a large change rate of the respiratory rate when a subject is at rest PA1 (ii) The change rate of the respiratory rate becomes small when the subject performs exercise of moderate intensity PA1 (iii) During intense exercise, regular breathing becomes impossible and the change rate of the respiratory rate becomes large
Autonomic training, also known as concentrated self-relaxation technique, is known to be helpful in promoting or restoring health by reducing tension. This type of training emphasizes placing the mind in a state of relaxation. However, the subject may become too focused on his efforts to relax, so that tension results instead. Breathing exercise has been developed as a part of autonomic training, to aid the subject in easing tension and shifting into a state of relaxation. For example, the subject chants a phrase such as "breath slowly" to himself repeatedly. As a result, the subject is able to enter a state of relaxation.
Various Eastern meditation and health methods emphasize breathing manner. For example, regularity of breathing in very important in Zen meditation which teaches the subject to regulate "the body, breathing and mind." Similarly, breathing exercise is one of the eight training steps in yoga. In addition, meditation employs abdominal breathing. These Eastern breathing methods share the common feature of training the subject to conduct abdominal breathing, in order to accomplish mental relaxation.
The change rate of the respiratory rate is known to decrease when the mind has been placed in a relaxed such as described above.
Considerable medical attention has also been directed on respiration during sleep. As a result, it is now known that there is a high likelihood of sudden death in the presence of apnea syndromes. In Eastern medicine, the heart rate of a healthy subject is viewed to be four times of his respiratory rate. A heart rate which is less than this is referred to as a slow pulse, while a heart rate which is greater than this is called a fast pulse. In these cases, it is very likely that the subject suffers from some sort of physiological illness. In other words, the respiration pulse rate ratio may become indicator of health. Moreover, the present inventors discovered that this respiration pules rate relationship is maintained during exercise.
Namely, subjects with slow pulse frequently suffer from parasympathetic dominance, asthma, autonomic imbalances, hynoteneion, and so on. On the other hand, subjects with fast pulse frequently suffer from high blood pressure, pneumonia, hepatitis, or other inflammatory diseases. Accordingly, if data can be obtained for pulse and respiratory rates during daily activities, it is possible to estimate the subject's state of health. In recent years, however, there has also been an increasing need to detect a subject's physiological state not only during daily activities, but also when the subject is active or exercising (pulse rate, arrhythmia, respiratory rate, etc.), such as in the case of scientific training or health monitoring of an athlete.
How the respiratory rate is measured is an important problem. Typical method for measuring the respiratory rate in a sedentary subject, such as a sick patient, include applying a band around the subject's chest or stomach, and then counting the number of expansions and contractions, or inserting a thermocouple in the subject's nostrils and counting the variation in the resistance value. However, employing such devices in a subject who is monitoring his daily health, or who is carrying out training would provide a considerable inconvenience.
A frequency analysis RR interval fluctuation in an electrocardiogram at rest reveals the presence of a component corresponding to the respiratory rate. Since the pulse wave is synchronized with the electrocardiogram, a component corresponding to the respiratory rate should be also included in the frequency analysis of the fluctuation pattern of the pulse wave cycle (or the pulse wave amplitude).
A device has been disclosed which measures the respiratory rate based on an electrocardiogram or pulse wave, by extracting this component. For example, Japanese Patent Application Show 62-22627 discloses calculating the respiratory rate by measuring continuous pulse intervals, measuring the cycle of fluctuation in these pulse intervals, and then taking the reciprocal of the cycle of fluctuation.
JPUA 451912 discloses a technique in which a first respiratory rate is detected based on the fluctuation in the envelope formed by the peak values of the pulse waveform or the cycle of fluctuation in the RR interval in the waveform of the electrocardiac waveform, a second respiratory rate is detected by detecting the up-and-down motion of the surface of the subject's trunk, and recording and displaying the lower of these two respiratory rates.
JPUA 4136207 discloses estimating the respiratory rate based on the fluctuation cycle in the amplitude of the pulse waveform, and calculating the average value of the pulse waveform (the wave in the low frequency component). By employing data obtained during the average value trend is small, the influence from swell or noise can be reduced.
JPA 6142082 discloses multiplying a subject's maximum blood pressure value and pulse rate which are successively obtained, and then calculating the respiratory rate based on the pulse cycle of the multiplied value. JPUB 622325 discloses a technique for determining the respiratory rate based on the cycle of fluctuation of a curved line connecting peak values in the pulse wave.
Accordingly, the present inventors hypothesized that the relationships described above were maintained during exercise, i.e., that the respiratory rate could be estimated based on an electrocardiogram or pulse wave during exercise. Clinical studies were carried out to test this hypothesis. As a result, it was understood that a component corresponding to respiratory rate is present in the frequency component of the pulse wave fluctuation or the RR interval fluctuation of an electrocardiogram during exercise.
However, in the case of an exercising subject, an electromyogram is superimposed on the electrocardiogram waveform, while a body motion component gets superimposed on the pulse wave. Since these components have a higher level than the components corresponding to the respiratory rate, calculations were carried out using an incorrect respiratory rate based on components obtained during exercise.
The present invention was conceived in consideration of the above circumstances, and has its objective of the provision of a physiological state measuring device which can accurately and easily measure a subject's respiratory rate, particularly during exercise. It is another objective of the present invention to provide a device which extracts the respiratory component from the pulse wave, and then guides the subject based on the rate of change in the extracted component, so that the subject enters a relaxed mental state.